Angiogenin (ANG), a member of the RNase family of proteins, induces blood vessel formation (angiogenesis). ANG has limited, but necessary, ribonuclease activity. Recently, an ANG/ribonuclease A hybrid protein (RNANG), displaying angiogenic activity, was created by replacing an exterior loop in RNase A with the corresponding loop from ANG. The purpose of the research presented in this proposal is to study the molecular basis for cellular recognition and binding, nuclear localization, and the cytotoxicity of ANG and RNANG. Site-directed mutagenesis will be used to create ANG/RNase A hybrid proteins of varying activity and specificity. The ability of the mutants to promote/inhibit angiogenesis in vivo will be assessed. Effects on cellular growth rate and cytotoxicity will be screened against a panel of endothelial and tumor cells selected by their ability to internalize RNANG. A fundamental understanding of angiogenic activity would ideally lead to developing a therapeutic control of blood vessel formation in tumor growth and metastasis.